Mobile communication is performed using the downlink (DL) from a radio communication base station apparatus (hereinafter abbreviated as “base station”) to radio communication mobile stations (hereinafter abbreviated as “mobile stations”), and the uplink (UL) from mobile stations to a base station.
The uplink and the downlink are associated with one another, and, for example, when ARQ (Automatic Repeat Request) is applied to downlink data, a mobile station feeds a response signal indicating the result of error detection about the downlink data, back to a base station using the uplink. A mobile station performs CRC (cyclic redundancy check) check on the downlink data, and, when CRC=OK (no error), feeds ACK (acknowledgment) back to a base station as a response signal, and, when. CRC=NG (error present), feeds NACK (negative acknowledgment) to the base station as a response signal. This response signal is transmitted to a base station using an uplink control channel such as a PUCCH (physical uplink control channel).
In addition, as shown in Non-Patent Literature 1, a base station transmits control information to report downlink data resource allocation results, to mobile stations. This control information is transmitted to mobile stations using downlink control channels such as PDCCHs (physical downlink control channels). Each PDCCH is allocated to one or more CCEs. When one PDCCH is allocated to a plurality of CCEs (control channel elements), this PDCCH is allocated to a plurality of consecutive CCEs. A base station assigns any of a plurality of PDCCHs to each mobile station, according to the number of CCEs (CCE aggregation size) required to report control information, maps control information to physical resources corresponding to CCEs (control channel elements) to allocate PDCCHs to, and transmits the mapped result.
In addition, studies are underway to associate CCEs and PUCCHs in order to efficiently use communication resources in the downlink. According to this association, each mobile station can determine PUCCHs used to transmit response signals from the mobile station, based on the CCEs corresponding to the physical resources to which control information directed to the mobile station is mapped.
In this way, there are associations between the uplink and the downlink, and therefore, when coexistence of a plurality of communication systems is desired, there is a problem that frequency resources run short if the uplink and the downlink are assigned to each of a plurality of communication system. In addition, when a new communication system is added to the band used to operate an old communication system, it is preferable to allow mobile stations in the old communication system to be used as is without change in the new system. As a method of solving the above-described problems, a frequency overlay system is proposed in Patent Literature 1.
With Patent Literature 1, when an old communication system and a new communication system exist together in order to improve efficiency of use of frequencies, the new system is designed to cover the frequency of the old communication system and performs frequency scheduling including the frequency of the old communication system. In addition, a design approach is adopted where correlation between preamble channels (reference signals) used in an old communication system and preamble channels (reference signals) used in a new communication system is low to improve accuracy of channel estimation. Moreover, different control channels are provided in an old communication system and a new communication system, individually, and transmitted in different frequency bands.